Resistor device



Patented Nov. 27, 1945 RESISTOR DEVICE Joseph J. Kleimack, Bayonne, andGerald L. Pearson, Millington, N. J., assignors to Bell TelephoneLaboratories,

Incorporated, New

York, N. Y., a corporation of New York Application January 29, 1942,Serial No. 428,654

7 Claims.

This invention relates to resistors and more particularly to resistorshaving a relatively high temperature coeflicient of resistance and lowthermal inertia.

Resistors, the resistance of which varies greatly with changes intemperature, have .for convenience been called thermistors," the termbeing derived from the words thermal" and resistor." Wherever the wordthermistor appears in this specification or the appended claims, it isintended to designate a resistor having the foregoing characteristic.Since most'of the known materials exhibiting thermistor characteristicsand at the same time having suflicient conductivity to render themuseful as circuit elements, comprise the semiconductors-having highnegative resistance-temperature coefficients, exemplary descriptions andexplanations in this specification may be made with reference to suchmaterials. It should be understood, however, that such electricallyconducting materials as may have a high positive resistance-temperaturecoeiificient, are not thereby excluded from the invention described andclaimed.

It has been found that a thermistor may be made to exhibit a negativeresistance characteristic. For example, the static volta e-currentcharacteristic curve of a negative resistance-temperature coefilcientthermistor risesto a voltage maximum and then falls oil. This decliningvoltage-current curve is an indication of the negative resistancecharacteristic. If a suificient direct current voltage is applied tosuch a thermistor to bias it beyond the voltage maximum point, thedevice will react to an applied alternating current as if its resistancewere negative, provided the frequency of the alternating current is nottoo high. A thermistor so used may 'be made to oscillate. The frequencyof scilla-- tion depends upon the heating and cooling ability of thethermistor, i. e., its thermal inertia. If the device is constructed tohave low thermal inertia it can be made t oscillate at relatively highfrequencies. P

It is an object of this invention to construct a thermistor having lowthermal inertia.

A feature of this invention resides in a resistor device constructed topresent a very small volume of thermistor material to the appliedcurrent.

Other and further objects and features of this invention will beunderstood more clearly and fully from the following detaileddescription with reference to the accompanying drawing in which ,wireheld against the boron coating.

Fig. 1 is a perspective view of one illustrative form of the device;

Figs. 2 and 3 are enlarged views showing in detail certain features ofthe device of Fig. l;

and

Fig. 4 is a plot showing static and dynamic characteristics of a typicalthermistor.

Referring to the drawing, I0 is the thermistor device mounted in theenvelope H and connected to terminal members or contact pins I2 by meansof conductors l3, l4 and IS, The device is built around a rod I6 mountedon conductors i4 and IS. The rod l6 may be made of hardened steel,tantalum, tungsten or other suitable material. The conductors l4 and I5are preferably secured, as by welding, to spaced portions of the rod IEto inhibit twisting thereof. The rod l6 has a, recess on one sideadjacent its midportion to receive an element H. The element l! is ofhard surface conductive material and may include material of highresistance-temperature coefficient.

As shown in the drawing, the element l1 comprises a fine wire, forexample, of tantalum or tungsten, coated with a thin film ofsemiconductive material such as boron. The film should be thin withrespect to the diameter of the wire so that the volume of metal is highas compared with the volume of semi-conductive material. The element i!may be secured in the recess of rod l6 by any of several suitablemethods of which welding has been found particularly satisfactory. Toprepare the element IT for weld ing, a small amount of thesemi-conductive materialmay be removed from the wire adjacent its end orif the material is boron small spots of platinum boride may be providedthereon. A small amount of semiconductive coating may be removed fromthe ends, of the wire by crushing as in a pair of pliers. Platinumboride spots may be made by playing a small flame on a platinum Anotherelement I8, which may be similar to element ll, may have a conductivelead 20 welded or otherwise secured in electrical connection to one endthereof. Element I8 is oriented at an angle, preferably a right angle,to element l1 and held in contact therewith by means of a spring 2|. Thespring may be flat and of clock spring steel or equivalent material.Adjacent the ends of the spring are two elongated openings 22 and 23. Inassembling the device the spring is bowed and the rod is is inserted inthe opening 22 and 23. A bent over portion 24 of the spring 2! issecured to the rod l6, e. g., by welding. The element It may beinsulated from the spring 2| by a member 25 of insulating material suchas a small piece of mica. Lead 20 from the element I8 is connected tothe conductor I! which is in turn connected to one of the contact pinsl2. Electrical connectionto the element I1 is by way of rod l6 andconductors I4 and I5 or either of them.\

The envelope ll may be filled with helium to increase the cooling rateof the device. This may be done by evacuating the envelope to aboutmillimeters of mercury and baking it at about 200 C. for approximatelyfifteen minutes to remove absorbed gases, etc. The device is then cooledand the helium introduced, the final pressure being about atmospheric orslightly thereunder.

' Various modifications may be made in the above-described structure toproduce similar resistor devices. Each of the elements I! and [8 mayhave a thin coating of hard, high resistance-temperature coefllcientmaterial or the coating may be provided on one only. For example, a bitof tungsten or tantalum wire may be welded in the recess of rod l6 asthe element H, or such a bit of wire may be used as the element l8. Ineither case the opposing element will have a thin coating ofsemiconductive material thereon. The recess in the rod is may be omittedand a semiconductively coated element l8 pressed directly against therod. Insuch aseen from an inspection of this figure, the curve of staticcharacteristic rises to a voltage maximum Em and then falls off. Thestatic characteristic is obtained by subjecting the thermistor toincreasing direct currents and measur ing the voltage for each current.

Dynamically, the alternating current resistance is negative in theregion beyond the voltage maxiimum Em for suillciently low frequencies.If a direct current of a value Ib greater than I0 (that currentcorresponding to Em) be applied to the thermistor, a superposedalternating current of" frequency approaching zero will trace out acurvevdob, approximating the static characteristic.

If the superposed current has a very high frequency, the thermal lag ofthe thermistor will prevent any change in temperature and hence inresistance from taking place during a single cycle. The voltage currenttrace therefore will be along the ohmic resistance line cod. Atintermediate frequencies, the superposed current will produce traces asshown at e, f and g in the order of increasing frequency. At lowfrequencies; the effective alternating current resistance is negative,at high frequencies it is p sitive and at intermediate'frequencies itmay be either positive or negative; thu for some critical i'requency itbecomes equal to zero. This latter is case the rod should be of amaterial that does not react with the semiconductive material. Forexample, if the semiconductive material were boron the rod l6 might beof tungsten or tantalum but not of steel. Whatever the-modification, thecontact portions should present to each other relatively hard surfacesto inhibit deformation thereof, they should present contact surfaces toprovide a very small area of contact and the layer of semiconductivematerial should be so thin that it is in contact with a relatively largemass of metal i. e., the metal core. The elements I! and i8 or either ofthem may be made of other materials, such as tungsten wire coated withuranium oxide or with a, mixture of nickel and manganese oxides, orother suitable high resistance-temperature coeflicient materials.Instead of employing helium in the envelope it may be evacuated to from,1 10 to 5x10" millimeters of mercury. As another alternative, theenvelope ll may be filled with a liquid which will not react with thevarious elements and which will not decompose at the operatingtemperature, i. e., about 300C. Mechanical stability, i. e., rigidityand freedom from twisting may be obtained by using a flat conductor inplace of the two conductors l4 and IS. The rod It may be made ofinsulating material, such as glass or porcelain, with the element I! orthe equivalent thereof embedded in oneside adjacent its mid-portion.Such a rod could be secured directl in the base of the envelope andconnection made to the element I! by means of a. conductive lead likelead 20. If the rod It were made of insulating material it would notbenecessary to insulate the element l8 from the spring 2 I.

An understanding of the operation of this device may be derived fromamore specific discussion of the negative resistance or declining,voltage current characteristic obtainable with thermistors. In Fig. 4there is shown a voltagecurrent plot of the static anddynamiccharacteristics of a thermistor having'a negative temperaturecoeflicient of resistance. As may be the maximum frequency at which thedevice may be made to oscillate. In the devices of this invention, thethermal lag is very low so that the frequency at which the resistance iszero is very high. For example, devices having very thin boron films asthe active thermistor material have been made to present a negativeresistance to alternating current up to about 60 kilocycles per second.

'The structure of a rapidly acting or high speed thermistor made inaccordance with this terial having relatively high thermal conductivity.Thus the small volume subjected to electrical current is quickly heatedand the heat is conducted away rapidly upon cessation of current flow.Such a device is capable of responding to rapidly fluctuating currentand is therefore available for a rather wide field of use. These devicesconnected in suitable circuits may be used as oscillators, modulators,amplifiers or filters.

Although specific embodiments of this invention have been described andillustrated it is to be understood that modifications may be madetherein within the spirit and scope of the appended claims.

What is claimed is:

1. A resistor device comprising a relatively stiff rod, a first membersecured centrally of said rod, a second member, spring means secured tosaid rod and embracing said second member to hold it between the'rod andspring and against said first member, said members being shaped andoriented to make substantially point contact with one another, bothmembers having relatively hard surfaces, at least one member including athin layer of high resistance-temperature coeiflcient material at thepoint of contact, and means for connecting each member to an electriccircuit.

2. A resistor device comprising a stiff elongated body, a first membercomprising a thin layer of high resistance-temperature coefiicientmaterial secured centrally of said body, a second member also comprisinghigh resistance-temperature 'coeiiicient material, a spring meanssecured to said body and embracing said second member to hold 7 itagainst said first member, said members having convex portions orientedto makesubstantially point contact with each other, and means forconnecting each member to an electric circuit.

3. A resistor device comprising a stiff metal rod having a recesscentrally thereof, a fine tantalum wire having a thin coating of boronthereon, said wire secured in said recess, a thin, flat spring of metalsecured at one end to said rod, which passes through an orifice adjacentthe other end thereof, a second boron coated tantalum wire orientedsubstantially at right angles to the first wire and held in place bysaid spring, means for insulating the second wire from the spring,electrical connectors secured respectively to the rod and the secondwire, an envelope enclosing the foregoing assembly, and a gas having arelatively high thermal conductivity within said envelope.

4. A resistor device comprising a conductive rod having a recess on oneside thereof intermediate its ends, a first fine wire coated with a thinfilm of relatively hard, high resistance-temperature coefiicientmaterial secured in said recess and substantially parallel to said rod,a second fine wire coated with relatively hard, highresistance-temperature coefiicient material, a flat spring havingelongated openings adjacent its ends, said spring being bowed with therod passing through the elongated openings, one end of the spring beingsecured to the rod, said second, coated, fine wire being between the rodand the central portion of the spring and urged toward the first wire bythe spring, the wires being relatively oriented at such an angle thattheir coatings are in substantially point contact, an envelope having aplurality of conductors sealed theretbrough, a least two of saidconductors being secured in spaced, substantially parallel relation tosaid rod', and means for electrically connecting one other of saidconductors to said second fine wire.

r "5. A resistor comprising arelatively rigid rod 01' conductivematerial, said rod having-a recess on'one side thereof, and intermediateits ends, a short section of cylindrical wire coated with a thin layerof a relatively hard resistance material secured in the recess and inelectrical connection with said rod, a bowed, fiat spring having spacedopenings therein, the spring being secured at one end to said rod, whichpasses through said open-' ings, a second cylindrical wire coated with athin layer of hard resistance material, between the rod and the centralportion of the spring and urged against the first coated wire, saidwires being approximately at right angles to each other, whereby theircoatings are in substantially point contact, insulating means betweenthe second coated wire and the spring and current concluctors securedrespectively to the rod and to the second wire.

6. An impedance device comprising a relatively stiff rod, a member,spring means secured to the rod and embracing the member to hold itagainst the rod, said rod and member having portions shaped and orientedto make substantially point contact with one another, at least one ofsaid portions including a thin layer of high resistance-temperaturecoefficient material at the point of contact, and means for connectingeach of said portions to an electric circuit.

7. An impedance device comprising a body, a member, resilient meanssecured to the body and embracing the member for holding the memberagainst the body, said body and member each having convex portionsoriented to make substantially point contact with each other, at leastone of said portions including a thin layer of highresistance-temperature coefficient material at the point of contact, andmeans for making electrical connection to each of said portions.

JOSEPH J. KLEIMACK. GERALD L. PEARSON.

